Valve seat

ABSTRACT

A ball valve seat ( 10 ) is disclosed which provides a temporary seal for a plug ( 40 ). The valve seat comprises a substantially cylindrical body ( 30 ) of a first volume, which defines a seating surface ( 34 ). A pressure differential is developed across the valve seat when the plug sealingly engages the seating surface. The body is formed of an elastic material which compresses from a first volume to a smaller second volume by application of a force on the plug, to provide a clearance which is greater than a plugging dimension of the plug, thus allowing passage of the plug downstream. After passage of the plug, the body returns to the first volume.

The present invention relates to valves used in downhole tools withinthe oil and gas industry and, in particular, a ball valve seat whichprovides a temporary seal for a travelling plug through the valve seat.

In the drilling, completion and production of oil and gas wells,downhole tools are mounted on a work string and run into a well bore toperform tasks or operations at locations within the well bore. A knownmethod of getting the tool to perform the task at the required time andlocation, is to drop a plug, typically in the form of a ball, throughthe work string, to engage with and actuate the tool. Such plugs arecarried with the fluid flow to the tool whereupon they encounter a valveseat and provide a sealed obstruction to the fluid flow path.

Commonly, shearable connectors, such as shear pins, are used incombination with the plug and valve seat to render the obstruction of afluid flow path reversible. In practice, the plug sealingly engages thevalve seat over a range of operating pressures. When a predeterminedfluid pressure threshold is exceeded, the pins shear, opening fluidpaths around the combination of the plug and valve seat. A disadvantageof this approach is that the tool must be designed with bypass channelswhich open around the plug and valve seat when the pins shear. Thesedesigns are expensive to manufacture and the channels can become blockedwith debris carried in the well bore fluid.

A further disadvantage of these designs is that once the plug is seated,the central bore of the work string is permanently obstructed. Thisprevents the passage of other strings such as wireline through the workstring.

To overcome this problem various valves have been designed with the aimof temporarily holding the plug while the tool is actuated and thenreleasing the plug to travel further through the work string. Deformableballs have been used which deform over a pressure threshold to squeezethrough the valve seat. A disadvantage of these deformable balls is thatthey are typically made of materials which can be susceptible to damageas the ball passes down the work string. If damaged they may not form aseal at the valve seat.

Releasable valve seats have been proposed which rely on a collet to holdthe ball temporarily. These seats can lack the effective seal betweenthe ball and seat.

Metal valve seats have also been proposed, for example in U.S. Pat. No.5,146,992. This presents an aluminium valve seat which is adapted forreceiving and temporarily sealingly engaging, a valve plug which ispositionable within the wellbore. The seat includes a sealing lip whichis adapted for sealingly engaging the valve plug and for substantiallyoccluding the passage of fluid from an upstream location to a downstreamlocation, wherein a pressure differential developed across the valveseat and plug operates to deform the sealing lip and allow passage ofthe valve plug downstream within the fluid conduit, when a predeterminedamount of force is applied thereto.

While this arrangement has the advantage of temporarily sealing at thevalve and allowing the plug to be later released, the valve seat has anumber of disadvantages. The main disadvantage is that once a plug haspassed through the seat, the valve seat has been deformed, providing awider clearance, so that a plug of similar or identical dimensions wouldnot seat within the valve, but pass therethrough. This means that thevalve seat can only be used once with a valve plug of a first dimension,and if a further occlusion of the fluid passage is required, eachconsecutive plug must have a greater plugging dimension. This requiresthe operator to be fully aware of the properties of the material usedand how it will behave under pressure and temperature to provide a plugwhich will have sufficient dimensions to make an effective seal on theseat, whilst still being able to deform the valve seat at a requiredpressure differential.

A further disadvantage of this invention is that the deformation takesplace primarily at a sealing lip, the sealing lip extending in thedirection of fluid flow. There is therefore a cavity behind the sealinglip into which the sealing lip moves or deforms. Debris and otherdeleterious material within the flow path can collect or gather behindthe sealing lip. This will then restrict the amount of deformation thatcan take place, and thus a plug can become stuck within the valve seat,and the assembly will have to be removed from the well bore atsubstantial cost.

It is therefore an object of the present invention to provide a valveseat for use with a plug in a downhole tool which can be repeatably usedfor the temporary occlusion of fluid flow through the tool by the use ofplugs having similar or identical plugging dimensions.

It is a further object of at least one embodiment of the presentinvention to provide a valve seat which is truly elastic, in that itdeforms within its own volume when pressure is applied, and returns toits original shape on release of the pressure.

According to a first aspect of the present invention, there is provideda valve seat, adapted for receiving a plug having a plugging dimension,for use in a fluid conduit of a downhole tool having an inner walldisposed about a central longitudinal axis, said inner wall defining acentral bore for passage of fluid from an upstream location to adownstream location, comprising:

a substantially cylindrical body having a first bore therethroughdefined by an inner surface of the body and the body being of a firstvolume;

a first clearance through said body, defined by a portion of said innersurface, which is smaller than the plugging dimension;

a seating surface located upon the inner surface facing upstream forsealingly engaging with said plug and substantially occluding passage ofsaid fluid from said upstream location to said downstream location;

wherein a pressure differential is developed across said valve seat whensaid plug sealingly engages said seating surface, applying force to saidseating surface;

said body being formed of an elastic material which compresses to asecond volume, smaller than said first volume, by application of saidforce to provide a second clearance to said body which is greater thanthe plugging dimension, and thus allows passage to said plug downstreamwithin said fluid conduit; and

wherein after passage of said plug, said body returns to said firstvolume with substantially said first clearance.

As the valve seat is elastic, it compresses under the force of the plugso that the outer dimensions of the body remain the same while the boreincreases radially to provide sufficient clearance for the plug to passthrough the seat. Further, as the valve seat returns to its same shapeand volume after passage of the plug, an identical plug can be droppedand the process repeated an indeterminate number of times.

Preferably the elastic material is a polymer.

More preferably the elastic material is a thermoplastic polymer. Suchthermoplastics include polyethylene and polypropylene.

The elastic material may be a thermoplastic polycondensate such as apolyamide or nylon.

The elastic material is preferably the thermoplastic polycondensate,polyetheretherketone (PEEK). Indeed, those skilled in the art willappreciate that materials which exhibit visco-elastic properties similarto polyetheretherketone would be acceptable. Polyetheretherketone isalso known under the trade names Arotone, Doctalex, Kadel, Mindel, PEEK,Santolite, Staver, Ultrapek and Zyex.

Preferably also the elastic material is a virgin material. Alternativelythe elastic material may include an additive. The additive may be glassgranules. Alternatively the additive may a fibre filler, such as carbon.The additive may be in a quantity of approximately 10 to 30%.

Preferably said inner surface is arcuate with said central longitudinalaxis. More preferably, said inner surface is convex to said centrallongitudinal axis. Preferably an apex of the convex defines the firstclearance. Such an arcuate profile provides a venturi feature as thegentle angle, both in and out through the valve seat will cause the plugto be sucked into the seat via the Bernoulli effect. Thus, the innersurface provides a gradual decrease to the first clearance which issymmetrical to the central longitudinal axis.

Preferably the inner surface is continuous with the inner wall. Thisprovides a non-turbulent fluid flow stream through the fluid conduit.

According to a second aspect of the present invention, there is provideda method of sealing a central bore of a downhole tool to temporarilyprevent passage of fluid from an upstream location to a downstreamlocation comprising:

-   -   (a) providing an elastic valve seat having a first volume and        defining a seat clearance within said central bore;    -   (b) providing a first plug having a first plugging dimension,        which exceeds said seat clearance of said elastic valve seat;    -   (c) seating said first plug against said elastic valve seat;    -   (d) developing, with said fluid, a differential pressure across        said elastic valve seat; and    -   (e) compressing said elastic valve seat at a pre-selected        pressure differential level to a second volume, smaller than        said first volume, to provide a clearance greater than the seat        clearance and allow passage of said first plug through said        elastic valve seat, wherein said elastic valve seat returns to        its first volume upon clearance of the first plug.

Preferably the method of sealing further comprises the steps of:

-   -   (f) providing at least one additional plug, which together with        said first plug constitutes a plurality of plugs having        substantially similar plugging dimensions;    -   (g) successively seating said plurality of plugs against said        elastic valve seat;    -   (h) successively developing, with said fluid, the same pressure        differential across said elastic valve seat; and    -   (i) successively compressing said elastic valve seat at the        pre-selected pressure differential level to provide clearance        for said plurality of valve plugs to pass through said elastic        seat and return the valve seat to its first volume between        successive seatings.

Advantageously, the method includes the step of sucking said plugtowards said valve seat, as said plug approaches said valve seat.

An embodiment of the present invention will now be described, by way ofexample only, with reference to the accompanying FIGURE.

FIG. 1 is a longitudinal section view through a portion of a downholetool, as would be used in the oil and gas industry. The valve seat 10 islocated within a recess 12 made from parts, generally indicated 14, ofthe downhole tool. Parts 14 comprise an upper section 16, mid section 18and a lower section 20. Sections 16, 18, 20 are provided for assemblypurposes of the tool, and will all move together as the seat 10 movesthrough the central bore 22.

The central bore 22 is located on a longitudinal axis 24 runningsymmetrically through the tool parts 14. The central bore 22 provides afluid conduit from upstream to downstream, upstream being locatedtowards the upper end 26 of the tool parts 14, and downstream beinglocated towards and extending from the lower end 28 of the tool parts14.

Recess 12 provides a substantially annular recess having a rectangularcross-section in the central bore 22. The recess 12 is made from theupper part 16 and mid part 18 of the tool parts 14. Located as a tightfit within the recess 12 is the valve seat 10.

The valve seat 10 comprises a unitary annular body 30 being donut orring shaped. In cross-section, as shown in the FIGURE, it provides twoopposite identical faces being mirror images. Each face 32 a,b comprisessubstantially planar upper and lower surfaces. A substantiallycylindrical outer surface abuts the recess base. An inner surface 34faces the central bore 22. Inner surface 34 is substantially cylindricalwith an arcuate profile on the longitudinal axis 24. The profile is madefrom a radius or arc with an apex or rise at a midpoint over the surface34. As illustrated, the body 30 defines a first volume.

The seat 10 is formed of polyetheretherketone, commonly referred to asPEEK. PEEK is a semi-crystalline polymer and falls within the class ofthermoplastic polycondensates. This material goes under the trade namesof PEEK, Arotone, Doctalex, Kadel, Mindel, Santolite, Staver, Ultrapekand Zyex. PEEK has a high tensile and flexural strength, high impactstrength and a high fatigue limit. Additionally it has a high heatdistortion temperature, high chemical resistance and high radiationresistance. It further has good electrical properties, good slip andwear characteristics and low flammability. The material can be injectionmoulded and may be formed with approximately a 10 to 30% addition ofglass granules. The addition of glass to PEEK increases its flexuralmodulus.

It is the visco-elastic properties of this material that make itsuitable, in that it can be compressed repeatedly and will always returnto its original volume and dimensions.

The typical mechanical properties of PEEK are:

Tensile stress at yield, at break   92 N/mm² Tensile modulus ofelasticity 3,600 N/mm² Flexural modulus 5 to 25 Gpa over −100 to 150° C.

Those skilled in the art will appreciate that other materials may beused for the formation of the valve seat, providing they havevisco-elastic properties which are around those found in PEEK. It islikely that these will come from polymers, e.g., polyamide (nylon),polyethylene, polypropylene and elastomers.

In use, the single piece valve seat 10 is located in a downhole toolbetween mating parts 16, 18. Preferably the seat 10 is located within arecess 12, such that the inner surface 34 aligns with the inner surfaces36,38 of the central bore 22 both above and below the seat 10. Thesurfaces 36,38, together with the inner surface 34, are provided withgentle angles and slopes, such that they provide a non-turbulent flow offluid through the central bore 22.

As it is located in the tool, the valve seat 10 is positioned within awellbore in a work string in which the tool is situated. The material ofthe seat 10 is non-erodable, thus chemicals and other flushingmaterials, such as muds, can be pumped through the bore 22 withoutdamage to the seat 10. Further, as the seat is formed of a relative softmaterial, it will not catch on any wireline or other tool insertedthrough the bore 22.

When a plug in the form of a ball 40 is released through the workstring, it will travel in the fluid through the central bore 22. Theball 40 is sized to have a dimension or diameter greater than theclearance through the seat 10 at the inner wall 34. In this way, as theball 40 travels through the bore 22 it will come to rest upon the seat10. This mating occurs at the upper edge of the seat 10 against asurface 42. The surface 42 may be referred to as a seating or sealingsurface, as a seal is formed due to the circumferential match of theball and the valve seat 10, as they come together. The ball 40 is thenseated in the valve seat 10.

Due the arcuate profile of the surface 42 on the inner surface 34, theball 40 will be sucked towards the seat 10, as it moves towards the seatdue to the Bernoulli effect. This prevents the ball from chattering orotherwise travelling back up the bore 22. Such phenomena exists if theball 40 may be made of a light weight material and the fluid pressurethrough the bore 22 is insufficient to carry the ball with sufficientforce to the seat. Additionally, the action of sucking the ball 40towards the seat 10 assists in tools which are located in horizontal ordeviated wells where gravity is not available to assist passage of theball 40.

With the ball 40 on the sealing surface 42 of the seat 10, fluid flowfrom upstream to downstream through the bore 22 is prevented. As fluidis pumped towards the ball 40 from upstream, a pressure differentialwill occur across the valve seat 10. The force upon the ball 40 will betranslated to a force on to the sealing surface 42 and to the body 30.This force will begin to compress the material of the seat 10.Compression will move the inner surface 34 radially into its own body30. The body 30 does not yield, expand, extrude or deform. This is notrequired as the material of the seat itself will compress into a smallervolume as the ball 40 pushes its way through the seat 10. This isevidenced by the fact that the recess 12 is of substantially the samedimensions as the body 30, so that there is no room for the body toyield, extrude or deform by expanding out of the central bore 22. As theseat 10 is compressed, the clearance through the seat 10 will increaseuntil it has the same dimensions of the ball 40, whereupon the ball 40will pass through the seat. Pressure upon the ball will now force theball 40 through the remainder of the bore 22 and a drop in the pressuredifferential will be noted at the well surface as fluid flow is restoredthrough the bore 22.

For the period of time that the ball is located on the sealing surface42 and fluid is occluded through the bore 22, the additional pressuredifferential not only forces the ball 40 through the seat 10, it willalso have the effect of forcing everything in line with this surface 42downstream. In the embodiment shown, it will mean that the parts 16, 18and 20 will be forced relatively downstream with respect to the workstring to which the tool is attached. This movement of the parts causesactuation of the tool. On release of the ball through the valve seat 10,if springs are located in the tool, these may reposition the parts 16,18, 20 on release of the ball. Thus the tool is both actuated andreturned to its initial configuration by the passage of a single ballthrough the valve seat.

As an example embodiment, a valve seat being provided with an outerdiameter of 82.55 mm, a depth of 30.75 mm, an arcuate profile radius of76.55 mm and a clearance at the input and output faces of 55.55 mm willoperate with a steel drop ball having a diameter of 52.43 mm.

The principle advantage of the present invention is that it provides aball valve seat which can be used a multiple number of times totemporarily halt the passage of a ball through the valve seat, the valveseat being self-healing and returning to its original dimensions afterthe passage of each ball. This allows the repetitive deployment ofidentical drop balls through a downhole tool to actuate the tool anychosen number of times.

A further advantage of the present invention is that the valve seat isshaped to provide a venturi effect as a plug or drop ball reaches thevalve seat. This effectively sucks the ball into the seat, providing afirm seating to the ball.

Various modifications may be made to the invention herein describedwithout departing from the scope thereof. For example, as discussed, anysuitable material having visco-elastic properties which exhibit thecompressible feature required of the invention could be used. Further,the relative dimensions of the valve seat may be altered to suit thesize of drop ball required, and the degree of space available to providea recess. Further, the radius of the arcuate surface of the seat withinthe bore can be selected to provide a required pressure differentiallevel at which the tool will activate.

1. A downhole tool for attachment to a work string, the downhole toolhaving an inner wall disposed above a central longitudinal axis, theinner wall defining a central bore for passage of fluid from an upstreamlocation to a downstream location; the downhole tool also having a valveseat, adapted for receiving a plug having a plugging dimension, for usein the central bore of the downhole tool, wherein the valve seatcomprises: a substantially cylindrical body having a first boretherethrough defined by an inner surface of the body and the body beingof a first volume; a first clearance through said body, defined by aportion of said inner surface, which is smaller than the pluggingdimension; a seating surface located upon the inner surface facingupstream for sealingly engaging with said plug and substantiallyoccluding passage of said fluid from said upstream location to saiddownstream location; wherein a pressure differential is developed acrosssaid valve seat when said plug sealingly engages said seating surface,applying force to said seating surface; said body being formed of anelastic material which compresses to a second volume, smaller than saidfirst volume, by application of said force to provide a second clearanceto said body which is greater than the plugging dimension, and thusallows passage of said plug downstream within said fluid conduit; andwherein after passage of said plug, said body returns to said firstvolume with substantially said first clearance; wherein the central boreof the downhole tool has an open upper end for receiving the plug, andwherein the tool is actuated and returned to its initial configurationby the passage of the plug through the valve seat.
 2. A valve seat asclaimed in claim 1, wherein the elastic material is a polymer.
 3. Avalve seat as claimed in claim 2, wherein the elastic material is athermoplastic polymer.
 4. A valve seat as claimed in claim 3, whereinthe elastic material is selected from the group consisting ofpolyethylene and polypropylene.
 5. A valve seat as claimed in claim 1,wherein the elastic material is a thermoplastic polycondensate.
 6. Avalve seat as claimed in claim 5, wherein the elastic material isselected from the group consisting of polyamide and nylon.
 7. A valveseat as claimed in claim 5, wherein the thermoplastic polycondensate ispolyetheretherketone (PEEK).
 8. A valve seat as claimed in claim 1,wherein the elastic material is a virgin material.
 9. A valve seat asclaimed in claim 1, wherein the elastic material includes an additive.10. A valve seat as claimed in claim 9, wherein the additive is glassgranules.
 11. A valve seat as claimed in claim 9, wherein the additiveis a fibre filler.
 12. A valve seat as claimed in claim 9, wherein theadditive is in a quantity of approximately 10 to 30%.
 13. A valve seatas claimed in claim 1, wherein said inner surface is arcuate withrespect to said central longitudinal axis.
 14. A valve seat as claimedin claim 13, wherein said inner surface is convex to said centrallongitudinal axis.
 15. A valve seat as claimed in claim 14, wherein anapex of the convex defines the first clearance.
 16. A valve seat asclaimed in claim 13, wherein an arcuate profile provides a venturifeature adapted to cause the plug to be sucked into the seat via theBernoulli effect.
 17. A valve seat as claimed in claim 16, wherein theinner surface provides a gradual decrease to the first clearance whichis symmetrical to the central longitudinal axis.
 18. A valve seat asclaimed in claim 1, wherein the inner surface is continuous with theinner wall.
 19. A method of sealing a central bore of a downhole tool totemporarily prevent passage of fluid from an upstream location to adownstream location comprising: providing an elastic valve seat withinsaid central bore, the elastic valve seat having a first volume anddefining a seat clearance and the central bore having an open upper endfor receiving a first plug; providing the first plug having a firstplugging dimension, which exceeds said seat clearance of said elasticvalve seat; seating said first plug against said elastic valve seat;developing, with said fluid, a differential pressure across said elasticvalve seat; and compressing said elastic valve seat at a pre-selectedpressure differential level to a second volume, smaller than said firstvolume, to provide a clearance greater than the seat clearance and allowpassage of said first plug through said elastic valve seat, wherein saidelastic valve seat returns to its first volume upon clearance of thefirst plug.
 20. A method as claimed in claim 19, further comprising thesteps of: providing at least one additional plug, which together withsaid first plug constitutes a plurality of plugs having substantiallysimilar plugging dimensions; successively seating said plurality ofplugs against said elastic valve seat; successively developing, withsaid fluid, the same pressure differential across said elastic valveseat; and successively compressing said elastic valve seat at thepre-selected pressure differential level to provide clearance for saidplurality of valve plugs to pass through said elastic seat and returnthe valve seat to its first volume between successive seatings.
 21. Amethod as claimed in claim 19, including the step of sucking said plugtowards said valve seat, as said plug approaches said valve seat.